It is known to provide an LED drive circuit which drives LEDs to emit light by applying a full-wave rectified waveform obtained by full-wave rectifying a commercial AC power supply to an LED array constructed by connecting a plurality of LEDs in series. If such a full-wave rectified waveform is simply applied to the LED array, the LEDs do not light when the voltage of the full-wave rectified waveform is lower than the threshold voltage of the LED array, and as a result, the LEDs become dim and produce a perceivable flicker. To address this, there is proposed a drive method in which the number of LEDs driven to emit light in the LED array is varied according to the voltage of the full-wave rectified waveform.
For example, patent document 1 discloses an LED drive circuit which comprises a commercial AC power supply, a bridge rectifier, an LED array comprising three LED groups, a bypass circuit comprising an FET Q1, a bipolar transistor Q2, and resistors R2 and R3, and a current limiting resistor R1.
It is also known to provide a lighting apparatus which detects a power ON/OFF operation by a wall switch or the like and controls the light output in multiple levels according to the number of ON/OFF operations performed.
For example, patent document 2 discloses a lighting apparatus which changes the brightness of lighting when power is turned on within a predetermined time after power is turned off. This lighting apparatus comprises a lamp load (L), an inverter circuit (1), an inverter control circuit (4), a power off detection circuit (2), and a time judging circuit (3), and the time judging circuit (3) controls the light output as a whole.
In the lighting apparatus disclosed in patent document 2, the inverter circuit (1) causes the lamp load (L) to light. The inverter control circuit (4) controls the operation of the inverter circuit (1) and changes the state of lighting of the lamp load (L). The power off detection circuit (2) detects the power being turned off by a switch (SW1). The time judging circuit (3) judges the length of time during which the power is off by a power off time detection signal and, if the length of time is not longer than a predetermined length of time, then controls the inverter control circuit (4) to select the state of lighting of the lamp load (L). In this way, the lighting apparatus controls the light output based on the ON/OFF operation of the switch.
In recent years, LED lamps using LEDs as light sources are being widely used, and there has also developed a need to incorporate a light output control function in such LED lamps.
For example, patent document 3 discloses an LED lamp whose light output is controlled by the ON/OFF operation of a wall switch. The LED lamp comprises a bridge rectifier (102), a toggle detector (74), a sustain voltage supply circuit (71), a counter (96), and an LED lighting driver (80).
The bridge rectifier (102) supplies a DC voltage by rectifying the AC voltage applied via the wall switch (98). The toggle detector (74) monitors the toggle operation of the wall switch (98). The sustain voltage supply circuit (71) supplies a sustain voltage so that the state and function of the counter (96) can be maintained after the wall switch (98) is turned off. The counter (96) counts the number of toggle operations performed. If the wall switch (98) is turned on/off after a predetermined time interval has elapsed, the counter (96) ignores such a toggle operation.
The LED lamp disclosed in patent document 3 generates a stable DC voltage with reduced ripple, applies the DC voltage to the LED with a duty cycle determined by the count value of the counter (96), and thereby controls the light output of the LED (light output control by pulse-width modulation). However, this LED lamp requires the use of a high-voltage withstanding, large-capacitance electrolytic capacitor when generating the DC voltage. This electrolytic capacitor is not only large in size, but its lifetime is reduced when it is used in a high temperature environment as in the case of an LED lamp. Furthermore, the complexity of the construction tends to increase, because various circuits such as an oscillator circuit for pulse-width modulation have to be incorporated in the lamp.
When driving an LED array constructed by connecting a plurality of LEDs in series, it is often the practice to connect in series to the LED array a current limiting device or circuit for limiting the current flowing to the LED array. The simplest way is to employ a resistor as the current limiting device, but it may not be desirable because the value of the current flowing to the LED array varies according to the applied voltage. In view of this, there are cases where a constant-current device or circuit is used as the current limiting device or circuit. If a constant-current diode is used as the constant-current device, the circuit can be made simple, but the disadvantage is that the constant-current diode itself has to be changed as it becomes necessary to adjust the value of the current to be flown to the LED array.
For example, patent document 4 discloses one that uses a three-terminal regulator as a constant-current circuit. In the light-emitting device driving circuit disclosed in patent document 4, the constant-current circuit (10) is connected in series with a light-emitting circuit (LED array) (3a) containing light-emitting devices (LEDs) (2) and, within the constant-current circuit (10), the voltage at the current output end of a current detecting resistor is fed back to the three-terminal regulator.
For example, patent document 5 discloses a circuit in which a voltage divided between resistors (13) (current detecting resistors) connected in series with a current adjusting circuit (12) (three-terminal regulator) is fed back as a control signal to the current adjusting circuit (12) in order to minimize the variation of LED brightness while minimizing the limiting resistance and reducing the amount of heat generated.
FIG. 27 is a circuit diagram of a prior art LED drive circuit 400.
The circuit configuration can be simplified by using a depletion-mode FET instead of the above three-terminal regulator. In view of this, the LED drive circuit 400 which incorporates a constant-current circuit constructed from a combination of a depletion-mode FET and a resistor will be described with reference to FIG. 27.
In FIG. 27, the LED drive circuit 400 includes a bridge rectifier 401, an LED array 403, and the constant-current circuit 404. A commercial power supply 402 is connected to input terminals of the bridge rectifier 401. The bridge rectifier 401 is constructed from four diodes 401a, and has a terminal G for outputting a full-wave rectified waveform and a terminal H to which the current is returned. The LED array 403 is constructed by connecting a plurality of LEDs 403a in series; the anode of the LED array 403 is connected to the terminal G of the bridge rectifier 401 and the cathode is connected to the drain of the depletion-mode FET 405 contained in the constant-current circuit 404. The constant-current circuit 404 is constructed by combining the depletion-mode FET 405 with a current detecting resistor 406. One end of the current detecting resistor 406 is connected to the source of the depletion-mode FET 405, and the other end is connected to the gate of the depletion-mode FET 405 as well as to the terminal H of the bridge rectifier 401.
The drain-to-source current of the depletion-mode FET 405 is determined by the gate-to-source voltage. Assume that the drain-to-source current increases; then, since the source voltage with respect to the gate voltage increases due to the effect of the current detecting resistor 406, feedback is applied in a direction that constricts the current flowing through the depletion-mode FET 405. On the other hand, when it is assumed that the drain-to-source current decreases, since the source voltage drops, feedback is applied in a direction that increases the current. In this way, negative feedback is applied in the constant-current circuit 404 which thus operates in a constant current mode.